1. Field of the Invention
The present invention relates to high temperature superconducting oxides, and, more particularly, to a process for improving the superconducting properties thereof.
2. Description of Related Art
The impurity OH.sup.-, in place of O.sup.- (c), blocks charge (hole) transfer, O.sup.- (c) to O.sup.= (c), necessary for superconduction in high temperature superconducting oxides. The removal of OH.sup.- (c) is a necessary step towards achieving a stable higher state of oxidation.
In a paper by D. P. Devor et al, "Hydroxyl impurity effects in YAG(Y.sub.3 Al.sub.5 O.sub.12)", Journal of Chemical Physics, Vol. 81 (No. 9), pp. 4104-4117 (1984), it was shown that OH.sup.- (c) was still present in significant amounts after crystal growth at .apprxeq.2,000.degree. C. High temperature superconducting materials, being processed at a much lower temperature (.apprxeq.900.degree. C.), are expected to have an even higher H-impurity content.
In a paper by L. E. Gorre et al, "Hydrogen impurities in oxides and halides", Materials Research Bulletin, Vol. 20, pp. 1441-1451 (1985), it was shown that when the free-OH.sup.- content in vitreous SiO.sub.2 was reduced, using the proper reactive atmosphere processing (RAP) oxidizing combination, the lattice contracted and the material became extremely radiation hard. It should be recalled that lattice contraction raises T.sub.c because that parameter has a negative coefficient with pressure.
In a paper by R. C. Pastor et al, "Reactive atmosphere processing of oxides", Materials Research Bulletin, Vol. 16, pp. 1413-1420 (1981), the authors studies degradation of rare-earth oxides, particularly La.sub.2 O.sub.3, by water vapor. They showed that the oxidizing combination of I.sub.2 /O.sub.2 stabilized La.sub.2 O.sub.3, prolonging the onset of H.sub.2 O-uptake by .apprxeq.4 hours (induction period). It is now common knowledge that high temperature superconducting oxides, such as YBa.sub.2 Cu.sub.3 O.sub.7, are easily degraded by water (vapor or liquid).
In a paper by R. C. Pastor et al, "Crystal growth in a reactive atmosphere", Materials Research Bulletin, Vol. 10, pp. 117-124 (1975), the authors drew attention to the reactivity of the halogen (X) for displacing OH.sup.- impurity by electron transfer, EQU X(g)+OH.sup.- (c).fwdarw.X.sup.- (c)+OH.sup.o .uparw.. (1)
A need remains for a process to improve the superconducting properties and stability of high temperature superconducting oxides.